1. Field of the Invention
The present invention relates to a control system for an internal combustion engine and, more particularly, to an engine control system which controls a rapid increase in engine speed when a transmission is shifted into a neutral range from in-gear ranges. Typically, an engine ignition timing is set so as to enable an engine to provide a desired output according to the amount of intake air. However, when a throttle valve, used to control the amount of intake air, is closed while a transmission clutch is disengaged, the engine may experience an abrupt rise in rotational engine speed. This is because an engine of the type having a fuel injection system is typically provided with a surge tank having an increased volume and located downstream of the throttle valve. An engine of this type will be supplied with air from the surge tank having the increased volume for a certain time period after closing the throttle. This allows an air-fuel mixture to continue to burn for the certain time period.
2. Description of Related Art
In order to prevent such an abrupt speed increase, it was thought that an ignition time, at which fuel ignition is produced, should be delayed or retarded so as to drop the engine speed. However, because any abrupt speed increase varies, depending upon the engine properties, and because the degree of abrupt speed increase changes due to the effects of aging, it is hard to definitely establish an ignition timing retardation for various types of engines. If, as is preferable, an ignition timing retardation is set so as to be suitably large for a relatively new engine, which has a burning performance which has not been seriously degraded due to aging, the same engine, if its burning performance does eventually become degraded due to aging, will be overcontrolled. This is due to the fact that a smaller abrupt speed increase will occur as the engine ages. Overcontrol of the engine in this manner can cause the rotational speed to fall low enough so that the engine stalls. If, however, an ignition timing retardation is made suitably small for an engine which has a burning performance which has been degraded due to aging, while the engine does not in fact suffer from a burning performance which has been degraded due to aging, it is difficult for the engine to control an abrupt increase in engine rotational speed in a satisfactory manner.
An engine control system, such as that which is known from, for instance, Japanese Unexamined Patent Publication No. 62-58,053, conventionally delays or retards an ignition time at which fuel ignition is made during a transmission shift. The engine control system of the above-mentioned publication is designed to regularly delay or retard an ignition time for the engine, which is equipped with an automatic transmission, so as to avoid shift shocks. Consequently, this engine control system is unable to control an abrupt increase in engine rotational speed when the transmission is shifted into a neutral range from in-gear ranges.